Minute quantities of contaminant in an electrochemical cell, such as a battery, can lead to premature failure of the cell. In many cases, contamination elevates the rate of self-discharge within the cell. This self-discharge produces relatively low levels of heat energy within the cell, typically on the order of 10-100 microwatts for an implantable medical type battery. Microcalorimetry techniques can be used to measure this heat energy, quantify the self-discharge rate, and identify a potentially contaminated cell.
Microcalorimetry instrumentation for measuring the low energy levels referenced above, is presently commercially available. Notwithstanding, the time required to perform a microcalorimetry measurement is typically 2 to 4 hours for a single cell. The extraordinarily long period needed to test a single cell is due to the difficulty in measuring the low energy levels associated with a faulty cell. Sensing low energy output against an ambient environment can only be done over a long period of time with conventional insulating and testing techniques. Consequently, conventional microcalorimetry techniques for screening cells have been prohibitively slow and expensive for use in production settings.
Reducing the time needed for testing electrochemical cells is complicated by the difficulty in differentiating low energy levels emitted from self-discharging cells from ambient environments. Thermocouples are incapable of measuring low energy levels in an ambient environment that masks self-discharging energy emissions.
While it is an option to thermally insulate electrochemical cells in a typical production environment, it is only viable if the temperature rise is relatively large, on the order of one degree Celsius or more. To achieve this for a cell with a surface area on the order of 10 cm2, which is roughly the surface area for a typical implantable medical battery, would require a thermal insulation R-factor on the order of 60 or greater. Even the best of modern insulation materials cannot provide this level of insulation for a specimen with such a small surface area.
Accordingly, there is a need for a microcalorimetry technique and system that would enable measurement of low level energy associated with self-discharge from electrochemical cells, such as implantable medical cells in a shortened period of time.